Level shifter with embedded logic and low minimum voltage

ABSTRACT

In one embodiment, a level shifter circuit may include a shift stage that also embeds transistors that implement a logic operation on two or more inputs to the level shifter. At least one of the inputs may be sourced from circuitry that is powered by a different power supply than the level shifter and circuitry that receives the level shifter output. Additionally, the level shifter includes one or more dummy transistors that match transistors the perform the logic operation, to improve symmetry of the level shifter circuit. In some embodiments, certain design and layout rules may be applied to the level shifter circuit to limit variation in the symmetry over various manufacturing variations.

This application is a continuation-in-part of U.S. patent application Ser. No. 11/610,236, filed on Dec. 13, 2006 now U.S. Pat. No. 7,652,504.

BACKGROUND

1. Field of the Invention

This invention is related to the field of integrated circuits and, more particularly, to level shifting in integrated circuits having multiple voltage domains.

2. Description of the Related Art

As the number of transistors included on a single integrated circuit “chip” has increased and as the operating frequency of the integrated circuits has increased, the management of power consumed by an integrated circuit has continued to increase in importance. If power consumption is not managed, meeting the thermal requirements of the integrated circuit (e.g. providing components required to adequately cool the integrated circuit during operation to remain within thermal limits of the integrated circuit) may be overly costly or even infeasible. Additionally, in some applications such as battery powered devices, managing power consumption in an integrated circuit may be key to providing acceptable battery life.

Power consumption in an integrated circuit is related to the supply voltage provided to the integrated circuit. For example, many digital logic circuits represent a binary one and a binary zero as the supply voltage and ground voltage, respectively (or vice versa). As digital logic evaluates during operation, signals frequently transition fully from one voltage to the other. Thus, the power consumed in an integrated circuit is dependent on the magnitude of the supply voltage relative to the ground voltage. Reducing the supply voltage generally leads to reduced power consumption. However, there are limits to the amount by which the supply voltage may be reduced.

Reducing the supply voltage often reduces the performance of the circuits supplied by that supply voltage. If some circuits in the integrated circuit are busy (and thus need to perform at or near peak operation), the supply voltage must generally remain at a relatively high level. One technique to avoid this is to divide the integrated circuit into voltage “domains” that are supplied by separate supply voltages that may be independently adjusted. That is, the supply voltage for circuits in a given voltage domain is the corresponding supply voltage. Thus, some voltages may be reduced (or even powered down completely) while others remain high for full speed operation.

Once voltage domains that may be at different levels are introduced, it is often required to level shift signals from one domain to another to ensure proper operation in the receiving voltage domain. If the supply voltage from the source voltage domain of a level shifter is powered down, all input signals may be reduced to ground voltage, which may result in an inaccurate signal being provided to the receiving voltage domain by the level shifter. Inaccurate operation may result.

SUMMARY

In one embodiment, a level shifter circuit may include a shift stage that also embeds transistors that implement a logic operation on two or more inputs to the level shifter. At least one of the inputs may be sourced from circuitry that is powered by a different power supply than the level shifter and circuitry that receives the level shifter output. Additionally, the level shifter includes one or more dummy transistors that match transistors the perform the logic operation, to improve symmetry of the level shifter circuit. In some embodiments, certain design and layout rules may be applied to the level shifter circuit to limit variation in the symmetry over various manufacturing variations.

BRIEF DESCRIPTION OF THE DRAWINGS

The following detailed description makes reference to the accompanying drawings, which are now briefly described.

FIG. 1 is a block diagram of one embodiment of an integrated circuit.

FIG. 2 is a block diagram of one embodiment of a memory circuit shown in FIG. 1.

FIG. 3 is a circuit diagram of one embodiment of a level shifter shown in FIG. 2.

FIG. 4 is a circuit diagram of a second embodiment of a level shifter shown in FIG. 2.

FIG. 5 is a flowchart illustrating one embodiment of a method.

FIG. 6 is a flowchart illustrating another embodiment of a method.

FIG. 7 is a circuit diagram of a third embodiment of a level shifter shown in FIG. 2.

FIG. 8 is a circuit diagram of a fourth embodiment of a level shifter shown in FIG. 2.

FIG. 9 is a circuit diagram of a fifth embodiment of a level shifter shown in FIG. 2.

FIG. 10 is a circuit diagram of a sixth embodiment of a level shifter shown in FIG. 2.

FIG. 11 is a circuit diagram of a seventh embodiment of a level shifter shown in FIG. 2.

FIG. 12 is a block diagram of one embodiment of a system.

While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that the drawings and detailed description thereto are not intended to limit the invention to the particular form disclosed, but on the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the present invention as defined by the appended claims.

DETAILED DESCRIPTION OF EMBODIMENTS

Turning now to FIG. 1, a block diagram of one embodiment of an integrated circuit 10 is shown. In the illustrated embodiment, the integrated circuit includes a plurality of logic circuits 12, a plurality of memory circuits 14, and a power control circuit 16. The logic circuits 12 are coupled to the memory circuits 14. The power control circuit 16 is coupled to the memory circuits 14 (specifically, via a V_(L) OFF signal). The logic circuits 12 are powered by a first supply voltage provided to the integrated circuit 10 (labeled V_(L) in FIG. 1). The memory circuits 14 and the power control circuit 16 are powered by a second power supply voltage provided to the integrated circuit 10 (labeled V_(M) in FIG. 1). In the illustrated embodiment, the memory circuits 14 are also powered by the V_(L) supply voltage, as will be explained in more detail for certain embodiments below. The integrated circuit 10 may generally comprise the logic circuits 12, the memory circuits 14, and the power control circuit 16 integrated onto a single semiconductor substrate (or chip).

The logic circuits 12 may generally implement the operation for which the integrated circuit is designed. The logic circuits 12 may generate various values during operation, which the logic circuits 12 may store in the memory circuits 14. Additionally, the logic circuits 12 may read various values on which to operate from the memory circuits 14. For example, in various embodiments, the memory circuits 14 may include memory used for caches, register files, integrated-circuit-specific data structures, etc. The memory circuits 14 may implement any type of readable/writeable memory. In an example below, an SRAM memory will be used. It is noted that, while the illustrated embodiment includes a plurality of logic circuits 12 and a plurality of memory circuits 14, various embodiments may include at least one logic circuit 12 and at least one memory circuit 14.

Generally, if a logic circuit 12 is to access a memory circuit 14, the logic circuit 12 may generate various control signals to the memory circuit 14. For example, the control signals may include an address identifying the memory location in the memory circuit 14 that is to be accessed, a read enable signal which may be asserted to perform a read, and a write enable signal which may be asserted to perform a write. For a read, the memory circuit 14 may output data to the logic circuit 12. For a write, the logic circuit 12 may supply data to the memory circuit 14 for storage.

By separating the supply voltage for the logic circuits 12 and the memory circuits 14, the supply voltage for the logic circuits 12 (V_(L)) may be reduced below the level at which the memory circuits 14 may operate robustly. The supply voltage for the memory circuits 14 (V_(M)) may be maintained at the minimum supply voltage that provides for robust memory operation (or greater, if desired). Thus, the V_(L) supply voltage may be less than the V_(M) supply voltage during use. At other times, the V_(L) supply voltage may exceed the V_(M) supply voltage during use (e.g. at times when higher performance is desired and higher power consumption is acceptable to achieve the higher performance). Alternatively, the V_(M) supply voltage may be increased to match the V_(L) supply voltage if the V_(L) supply voltage would otherwise exceed the V_(M) supply voltage.

In one embodiment, the V_(L) supply voltage may even be powered down (that is, reduced to the ground reference) while the V_(M) supply voltage remains active to retain data in the memory circuits 14. The power control circuit 16 may monitor various inputs (internal, illustrated by arrow 17, and/or external, illustrated by arrow 18) to determine that the V_(L) supply voltage is to be powered down. In one embodiment, the power control circuit 16 may determine that the V_(L) supply voltage is to be powered down and may issue a power down request to an external power source such as a voltage regulator (not shown in FIG. 1) that supplies the V_(L) voltage. In other embodiments, internal or external inputs may indicate that the power down is to occur, and the power control unit 16 may detect the event by monitoring the inputs. In either case, the power control unit 16 may assert the V_(L)OFF signal to the memory circuits 14. The V_(L)OFF signal may be active high (where the asserted state indicating that the V_(L) voltage is to be powered off is the logical one state) or active low (where the asserted state is the logical zero state). The deasserted state is the opposite of the asserted state in either case. Various level shifters in the memory circuits 14 may use the V_(L)OFF signal to assert a predetermined output to other circuitry in the memory circuits 14, as described in more detail below.

In one embodiment, the integrated circuit 10 may support operation at very low V_(L) supply voltages. For example, in one embodiment, a V_(L) supply voltage as low as 0.55 volts may be supported. Various circuitry may be optimized to ensure correct operation at the low V_(L) supply voltages. Specifically, the above-mentioned level shifters may be optimized for such operation. The inventors have found that using level shifter circuits that are symmetrical (or as near to symmetrical as possible) may provide for lower supply voltage operation than asymmetrical level shifters. However, in some embodiments, the level shifter circuits may receive the V_(L)OFF signal and may provide a predetermined output in response to the V_(L)OFF signal. The circuitry to manage the V_(L)OFF signal and to level shift the input signal of the level shifter to the output tends to be asymmetrical. To improve the symmetry of the circuit, dummy transistors may be inserted into the circuit.

A circuit may be symmetrical, in general, if the circuit provides similar electrical properties on its output for a rising transition on a given input and a similar falling transition on that input (e.g. similar slope, voltage change, etc.). The electrical properties may include delay, rise/fall time, etc. In one embodiment, level shifter circuits may include a shift stage and an output stage. The shift stage may generally include a pair of “legs”, which may be asymmetrical to each other with the inclusion of transistors for the V_(L)OFF signal. To restore symmetry, dummy transistors may be inserted in the leg having fewer transistors. More particularly, each leg may have the same arrangement of transistors, with the same sizing parameters for corresponding transistors in the two legs. One leg may include n-type metal-oxide-semiconductors (NMOS) that respond to a rising edge of a given input signal (and p-type MOS (PMOS) transistors that respond to the falling edge of the given input). The other leg may include NMOS transistors that respond to a rising edge of a complement of the given input signal (and PMOS transistors that respond to the falling edge of the given input signal).

Generally, a supply voltage may be a voltage provided to a circuit to power the circuit, providing the electrical energy to permit the circuit to generate one or more outputs responsive to one or more inputs. At various points herein, supply voltages may be referred to as being greater than or less than other supply voltages. That is, the magnitude of the voltage may be greater than (or less than) the magnitude of the other voltage.

Turning now to FIG. 2, a block diagram of one embodiment of a memory circuit 14A is shown. The memory circuit 14A may be one of the memory circuits 14. Other memory circuits 14 may be similar. In the embodiment of FIG. 2, the memory circuit 14A includes a level shifter circuit 20, a set of word line driver circuits 22, a memory array 24, a clock gater circuit 26, and a control signal generator circuit 28. The level shifter 20 and the word line drivers 22 are supplied by the V_(M) supply voltage. The memory array 24 and the control signal generator 28 are supplied by both the V_(M) and the V_(L) supply voltages. The clock gater 26 is supplied by the V_(L) supply voltage. The clock gater 26 is coupled to receive a clock input (gclk) and one or more enable inputs (En) from the logic circuits 12. The output of the clock gater 26 (clk) is coupled as an input to the level shifter 20, which is also coupled to receive the V_(L)OFF signal. The level shifter 20 is also configured to generate a clock output (clk_e) to the word line drivers 22. The word line drivers 22 are further coupled to receive one or more address inputs (Addr inputs) from the logic circuits 12. The word line drivers 22 are configured to generate a set of word lines to the memory array 24 (WL0 . . . WLN). The memory array 24 is further coupled to receive data (Din) and provide data (Dout) to/from the logic circuits 12. Additionally, the memory array 24 is coupled to receive various control signals from the control signal generator 28. For example, the control signals may include a write enable (WE) signal and a read enable (RE) signal. The control signals may also include a precharge (PChg) signal, and any other desired control signals. The control signal generator 28 may generate the control signals for the memory array 24 from corresponding control inputs from the logic circuits 12, and may level shift control signals, in some embodiments. Thus, the control signal generator 28 may receive the V_(L)OFF signal as well and may include one or more level shifters similar to the level shifter 20.

The memory array 24 may comprise a plurality of memory cells that are supplied by the V_(M) supply voltage. However, the memory circuit 14A is designed to provide for access to the memory array 24 by the logic circuits 12, even if the logic circuits 12 are supplied with a V_(L) supply voltage that is less than the V_(M) supply voltage. Each memory cell is activated for access (read or write) by one of the word lines WL0 . . . WLN coupled to that memory cell. One or more memory cells coupled to the same word line form a “word” for access in the memory array 24. That is, the bits of the word may be read/written as a group. The width of the word may thus be the width of the Din and Dout signals from the memory array 24.

Since the memory cells are supplied by the V_(M) supply voltage, the word lines may also be supplied by the V_(M) supply voltage. That is, when a word line is asserted high, the word line may be at approximately a V_(M) voltage. Thus, the word line drivers 22 are supplied with the V_(M) supply voltage.

The word line drivers 22 activate a given word line based on address inputs from the logic circuits 12. The address identifies the word in the memory array 24 to be accessed for a given access generated by the logic circuits 12. In some embodiments, the logic circuits 12 may include circuits that partially or fully decode the address, and the address inputs may be the partially or fully decoded address. Alternatively, the word line drivers 22 may implement the full decode function and the address inputs may encode the address. Generally, each different address causes a different word line WL0 to WLN to be asserted.

Since the word line drivers 22 are supplied with the V_(M) supply voltage, inputs to the word line drivers 22 that are coupled to the gates of p-type metal oxide semiconductor (PMOS) transistors in the word line drivers 22 may be driven to a V_(M) voltage when driven high (to ensure that the PMOS transistors, which are supplied with a V_(M) supply voltage, are fully turned off when the gate is driven high). That is, if the gate of the PMOS transistor is driven to a voltage less than the V_(M) supply voltage on its source, the gate to source voltage of the PMOS transistor is still negative and thus the PMOS transistor may still be active even though it is logically intended to be inactive. If the word line drivers 22 were designed with static complementary MOS (CMOS) circuits, each input would be coupled to the gate of a PMOS transistor and would be driven to a V_(M) voltage when driven high. In one embodiment, the word line drivers 22 may be implemented with dynamic logic gates. Thus, the clock signal that precharges the circuit (clk_e) is coupled to the gate of a PMOS transistor and may be driven to a V_(M) voltage. Other signals, coupled to the gates of n-type MOS (NMOS) transistors, may be driven with the V_(L) voltage. Thus, the address inputs from the logic circuits 12 may be provided directly to the word line drivers 22 (without level shifting).

The level shifter 20 is configured to generate the clk_e signal responsive to the clk signal from the clock gater 26. If the clock is enabled for the current clock cycle, the level shifter 20 may generate the clk_e signal by level shifting the input clk signal such that the high assertion of the clk_e signal is at a V_(M) voltage.

Additionally, the level shifter 20 may be designed to provide a predetermined voltage level on the output signal (clk_e, in FIG. 2) if the V_(L) supply voltage is to be powered down, independent of the input signal to the level shifter 20. Since the input signal is generated by circuitry powered by the V_(L) supply voltage (and thus is powered down), the input signal may be at a ground voltage (or may even float at indeterminate levels). Furthermore, for the embodiments of FIGS. 3 and 4, the inverter 56 may be powered by the V_(L) supply voltage, and thus the input signal and its inversion are not complements of each other when the V_(L) supply voltage is powered down. By providing the predetermined voltage level during such time periods, the level shifter 20 may provide predictable behavior for the receiving circuitry. If predicable behavior was not provided, various undesired effects could occur. For example, for the embodiment of FIG. 2 (in which the level shifter provides signals for the memory circuitry), data in the memory could be accidentally overwritten. Or, circuitry that normally is not active at the same time (e.g. bitline prechargers and word line drivers) could be active at the same time and thus cause contention, which could damage the memory circuits and/or cause undesirable power consumption. Still further, when the V_(L) supply voltage is powered up again, unpredictable logic circuit initializations and/or transitions on the input signals to the level shifter may occur, but the forced value on the output may prevent such unpredictable behavior from reaching the memory circuits. In the present embodiment, the assertion of the V_(L)OFF signal indicates that the V_(L) supply voltage is to be powered down, and the V_(L)OFF signal may remain asserted while the V_(L) supply voltage is off, in this embodiment.

The predetermined voltage level may be the level that is considered “safe” for the receiving circuitry. For example, in the case of FIG. 2, the clk_e signal is coupled to the gate of a PMOS transistor in the word line drivers 22, as mentioned above. Accordingly, the safe voltage would be the high (V_(M)) voltage, so that the PMOS transistor is inactive. In other cases, the safe voltage may be the low (ground) voltage, or any other desired voltage level. Various level shifters may implement various predetermined voltage levels, even in the same design, dependent on the safe voltage levels for the receiving circuitry.

Generally, a level shifter circuit may be a circuit configured to level shift an input signal to produce an output signal. Level shifting a signal may refer to changing the high assertion of the signal from one voltage to another. Level shifting may be performed in either direction (e.g. the voltage after level shifting may be higher or lower than the voltage before level shifting). In some embodiments, the low assertion may remain the ground voltage supplied to the integrated circuit 10 (not shown in the figures, often referred to as V_(SS)).

The clock gater 26 generates the clk signal responsive to the En signal (or signals) and the gclk signal (similar to the discussion above for the level shifter). If the En signal (or signals) indicate that the clock is enabled for the current clock cycle, the clock gater 26 may generate the clk signal responsive to the gclk signal. If the En signal (or signals) indicate that the clock is disabled, the clock gater 26 may hold the clk signal steady at a low level (ground). In other implementations, the clock gater 26 may hold the clk signal steady at a high level (V_(L)) if the En signal (or signals) indicate that the clock is disabled.

As mentioned above, the memory circuit 14A is designed to provide read/write access to the memory array 24 even if the V_(M) supply voltage is higher than the V_(L) supply voltage. The level shifter 20 level-shifting input signals and the word line drivers 22 operating at the V_(M) voltage provide the start of an access. The Din and Dout signals provide the data in (for a write) or the data out (for a read), and thus are in the V_(L) domain used by the logic circuits 12 in this embodiment. The memory array 24 may also be supplied with the V_(L) voltage, and may be configured to operate with the Din and Dout signals in the V_(L) domain. In other embodiments, the Din and Dout signals may be level shifted between the V_(L) and V_(M) domains, or only the Din signals may be level shifted and the Dout signals may be in the V_(M) domain.

In one embodiment, at least the sense amplifier (senseamp) circuits in the memory array 24 that sense the bits read from the memory cells are supplied with the V_(L) voltage. Thus, the senseamps may also provide a level shift to the V_(L) domain for the Dout signals. In another embodiment, the senseamp circuits may be supplied with the V_(M) voltage and the Dout signals may be in the V_(M) domain. In another implementation, the bit lines coupled to the memory cells to communicate the bits into and out of the memory cells may be in the V_(L) domain and thus other circuitry that is coupled to the bit lines may be supplied with the V_(L) supply voltage (except for the memory cells themselves).

As mentioned previously, signals in the V_(L) domain that are coupled to the gates of PMOS transistors that are supplied by the V_(M) supply voltage may be level shifted. Thus, in various embodiments, some of the control signals provided to the memory array 24 may be level-shifted. The control signal generator 28 may provide the level shifting, as needed, in various embodiments. If a given control signal is not level shifted, the control signal generator 28 may generate the control signal using circuitry supplied by the V_(L) supply voltage. If a given control signal is level shifted, the control signal generator 28 may include a level shifter to shift to the V_(M) domain. The level shifters in the control signal generator 28 may be similar to the level shifter 20 and may respond to an assertion of the V_(L)OFF signal by providing appropriate safe voltages on their outputs.

Turning now to FIG. 3, a circuit diagram of one embodiment 20 a of the level shifter 20 is shown. In the embodiment of FIG. 3, the level shifter 20 a includes a shift stage comprising transistors T₁-T₉ and T₂₀-T₂₂, and an output inverter comprising transistors T₁₁-T₁₃. T₃ has a source coupled to the V_(M) supply voltage, a gate coupled to a node N1, and a drain coupled to the source of T₄. The gates of T₄ and T₅ are coupled to receive an input signal (In), which may be the output of the NAND gate 30 in FIG. 2. Generally, however, any input signal may be used. The drains of T₁, T₄, T₅, and T₂₀ are coupled to the node N2. The source of T₅ is coupled to the drain of T₆, and the source of T₆ is coupled to ground. The gate of T₆ is coupled to an inversion of the V_(L)OFF signal, output from the inverter 50. The output of the inverter 50 and is also coupled to the gate of T₁, which has its source coupled to the V_(M) supply voltage. The In signal is input to an inverter 56 which has its output coupled to the gates of T₈ and T₉. The source of T₉ is coupled to the drain of T₂₁, which has its source coupled to ground and its gate coupled to the V_(M) supply voltage. The drains of T₈ and T₉ are coupled to the node N1. The source of T₈ is coupled to the drain of T₇, which has its source coupled to the V_(M) supply voltage. The gate of the transistor T₇ is coupled to the node N2. The node N1 is the output of the shift stage and supplies the input to the output inverter. The gates of T₁₁ and T₁₃ are coupled to the node N1, and the drains of T₁₁ and T₁₂ are coupled to the output signal (e.g. the clk_e signal in FIG. 2). The source of T₁₁ is coupled to the V_(M) supply voltage. The source of T₁₂ is coupled to the drain of T₁₃, which has its source coupled to ground. The gate of T₁₂ is coupled to the V_(L) supply voltage. The drain of T₂ is coupled to the node N1, and the source of T₂ is coupled to ground. The gate of T₂ is coupled to the output of an inverter 52, which has its input coupled to the output of the inverter 50. The drain of T₂₂ is coupled to the node N1, and the gate and source of T₂₂ are coupled to the V_(M) supply voltage. The transistor T₂₀ has its gate and source coupled to ground.

Operation of the shift stage will first be described, assuming that the V_(L)OFF signal is deasserted. When the input signal (In) transitions from low to high, T₅ is activated and begins discharging node N2. T₄ is also deactivated by the input signal transition, isolating the node N2 from T₃. As the node N2 discharges, T₇ activates and begins charging node N1 to the V_(M) supply voltage (T₈ is also activated, and T₉ is deactivated, by the transition to low on the output of the inverter 56 due to the transition high of the input signal). Accordingly, N1 results in the same logical state as the input signal, at the V_(M) supply voltage. When the input signal transitions from high to low, the output of the inverter 56 transitions from low to high and T₉ is activated. T₉ begins discharging the node N1. T₈ is also deactivated by the input signal transition, isolating the node N1 from T₇. Thus, the node N1 is discharged to ground. As the node N1 discharges, T₃ activates and begins charging node N2 to the V_(M) supply voltage (T₄ is also activated by the transition to low of the input signal), thus deactivating T₇.

In this embodiment, the level shifter 20 a also provides a forced, predetermined voltage on the output if the V_(L)OFF signal is asserted (high). If the V_(L)OFF signal is asserted, T₂ is activated and discharges node N1 to ground. T₁ is also activated and charges node N2 to the V_(M) voltage. This forces a value of ground on node N1 (or a V_(M) voltage on the output signal). T₆ is deactivated, preventing T₅ from affecting the node N2. T₇ is deactivated as the node N2 charges. Thus, the output node N1 may be held steady at ground if the V_(L)OFF signal is asserted, independent of the state of the input signal. The output signal (Out) may thus be held at the V_(M) voltage through the inverter formed from transistors T₁₁, T₁₂, and T₁₃. If the V_(L)OFF signal is deasserted (low), T₆ is activated via the output of the inverter 50 and thus the shift stage may operate as described above. T₂ is deactivated via the output of the inverter 52, and T₁ is also inactive. Transistors T₁, T₂, and T₆ establish voltages on the internal nodes N1 and N2 that are consistent with the desired output signal level.

The operation of the active transistors T₁-T₉ in the shift stage has been described. However, if only the transistors T₁-T₉ were included, the shift stage would be asymmetrical. For example, the discharge of node N2 is through T₅-T₆, and the discharge of node N1 would be only through T₉. Additionally, node N2 has a PMOS load for the V_(L)OFF signal, while the node N1 has an NMOS load for the V_(L)OFF signal. To provide a more symmetric circuit, the transistors T₂₀-T₂₂ are inserted into the level shifter 20 a. The transistors T₂₀-T₂₂ may be referred to as “dummy transistors”. Dummy transistors may not be required for the functionality of the circuit, but rather may be provided to match functional transistors elsewhere in the circuit. For example, the transistor T₂₀ may match the transistor T₂; the transistor T₂₁ may match the transistor T₆; and the transistor T₂₂ may match the transistor T₁. A dummy transistor may match a corresponding other transistor if the dummy transistor provides similar electrical performance as the other transistor (e.g. load, current if active, etc.). For example, in one embodiment, the dummy transistor may sized the same as the corresponding other transistor.

The dummy transistors may have their gates tied to a static voltage. For example, the dummy transistors may be coupled to the power supply voltage (V_(M)) or ground. The dummy transistors may be tied in the “on” state (active) or the “off” state (inactive). If the dummy transistors are provided to present a load, but do not need to be on for functionality, the dummy transistors may be in the “off” state, which conserves power (e.g. the transistors T₂₀ and T₂₂). If the transistors need to be on for functionality (e.g. transistor T₂₁), the dummy transistor may be in the “on” state.

A static voltage may refer to a voltage which is nominally held at a fixed level during use. That is, the voltage is not a signal that is expected to vary during use. In actuality, there may be some variation (e.g. due to noise, voltage droop under load, etc.).

The shift stage may be viewed as two legs of circuitry. One leg includes transistor T₃-T₆, T₁, and T₂₀. The other leg includes T₂, T₇-T₉, and T₂₁-T₂₂. The legs are symmetrical with the addition of the dummy transistors T₂₀-T₂₂. As shown in FIG. 4, the leg including the transistors T₃-T₆, T₁, and T₂₀ responds to the input signal (In) and the complement of the V_(L)OFF signal. Conversely, the leg including the transistors T₂, T₇-T₉, and T₂₁-T₂₂ responds to the V_(L)OFF signal and the complement of the input signal.

While the above description describes operation in response to the input signal and the V_(L)OFF signal, the level shifter 20 a may generally be viewed as embedding logic functionality. For example, the functionality of the level shifter 20 a is the AND of the input signal and the V_(L)OFF signal. Other embodiments may embed any functionality, for any set of two or more input signals. For example, NAND, NOR or OR functionality could be used. At least one of the input signals is generated by circuitry in the V_(L) logic domain and the level shifter 20 a may implement level shifting along with the logical operation.

T₄ and T₈ may limit power dissipation during transition, by isolating the nodes N2 and N1, respectively, from T₃ and T₇, respectively. T₃ and T₇ may be delayed in deactivating with respect to the activation of T₅ and T₉, respectively, since T₃ and T₇ are deactivated through the charging of nodes N1 and N2, respectively. By isolating T₃ and T₇ from their respective nodes N2 and N1 when T₅ and T₉ are activated, T₃ and T₇ may be prevented from fighting the discharge of their respective nodes N2 and N1. T₄ and T₈ are optional and may be deleted in other embodiments. In such embodiments, the drains of T₃ and T₇ may be coupled to the drains of T₅ and T₉, respectively.

The output inverter provides output buffering, which may permit the transistors T₁-T₉ to be smaller. The output inverter is optional and may be eliminated in other embodiments. T₁₁ and T₁₃ provide the inversion. In the illustrated embodiment, the transistors T₁₂ is provided to aid in matching the delay of the level shifter 20 a to other circuitry supplied by the V_(L) supply voltage (e.g. the clock gater 26). The transistor T₁₂ is optional and may be eliminated in other embodiments that have the inverter formed from transistors T₁₁ and T₁₃.

The embodiment of FIG. 3 provides a logical one output (V_(M) voltage on the output signal) if the V_(L)OFF signal is asserted. The embodiment of FIG. 4 is an example of a second embodiment of the level shifter 20 (level shifter 20 b) that provides a logical zero output (ground on the output signal).

The embodiment of FIG. 4 includes a shift stage comprising T₃-T₅, T₇-T₉, and T₁₄-T₁₉, and an output inverter comprising T₁₁ and T₁₃. The transistor T₁₂ has been eliminated in this embodiment, since the output signal (Out) is ground when V_(L) is powered down in this embodiment. In order to drive the ground voltage via T₁₃, T₁₂ is eliminated. T₃-T₅, T₇-T₉, and T₁₀-T₁₁ and T₁₃ are coupled to each other and to the nodes N1 and N2 in a manner similar to the embodiment of FIG. 2 (except that the transistor T₁₂ has been removed and thus the drain of T₁₃ is coupled to the drain of T₁₁). T₃-T₅, T₇-T₉, T₁₀-T₁₁, and T₁₃ operate similar to the description of FIG. 3 responsive to the high and low inputs on their gate terminals. Comments regarding portions that are optional may be similar to the description of FIG. 3 as well. However, the source of T₉ is coupled to the drain of a transistor T₁₅, which has its source coupled to ground and its gate coupled to the output of the inverter 52, which has its input coupled to the V_(L)OFF signal. A matching dummy transistor T₁₉ is provided, with its drain coupled to the source of the transistor T₅ and its gate tied to the power supply voltage (V_(M)) so that T₁₉ is in the on state during use. The gate of a transistor T₁₆ is coupled to the V_(L)OFF signal as well, and has its drain coupled to the node N2 and its source coupled to ground. A matching dummy transistor T₁₈ is provided on the node N1, with its gate tied to ground and thus the transistor T₁₈ is in the off state. A transistor T₁₄ has its drain coupled to the node N1 and its source coupled to the V_(M) supply voltage. The gate of T₁₄ is coupled to the output of the inverter 56. A matching dummy transistor T₁₇ is coupled to the node N2, and has its gate tied to the supply voltage V_(M), keeping it in the off state. Thus, if the V_(L)OFF signal is asserted (high), the transistor T₁₆ discharges the node N2 to ground, and the transistor T₁₄ charges the node N1 to the V_(M) supply voltage (and the transistor T₁₅ prevents the transistor T₉ from having an effect on the node N1). The output signal is thus a logical zero (ground voltage) in response to assertion of the V_(L)OFF signal, independent of the input signal. Transistors T₁₄, T₁₅, and T₁₆ establish voltages on the internal nodes N1 and N2 that are consistent with the desired output signal level.

It is noted that the transistors in the embodiments of FIGS. 3 and 4 may be PMOS or NMOS transistors, using the standard symbols for such transistors. That is, a transistor with a gate terminal having an open circle (e.g. the transistor T₁) is a PMOS and a transistor with a gate terminal having no open circle (e.g. the transistor T₂) is an NMOS.

As can be seen in the embodiments of FIGS. 3 and 4, various transistors may have gate terminals coupled to receive the V_(L)OFF signal or its complement (inverse). Accordingly, the transistors may generally be controlled responsive to the V_(L)OFF signal. It is noted that, while various nodes/signals are described as being charged/discharged to a given voltage level, there may be some variation in voltage levels on nodes. Accordingly, a node may generally be charged/discharged (or driven) to approximately the given voltage level.

It is noted that, while logic circuits that may be powered down and memory circuits that may remain powered, with level shifters as described above in between, are shown in this embodiment, other embodiments may be used in other fashions. Generally, there may be source circuitry powered by one supply voltage (that may be powered down) that supplies signals to level shifters and receiving circuitry powered by another supply voltage that receives the level shifted outputs from the level shifters (and that may remain powered when the source circuitry's supply voltage is powered down). Alternatively, the level shifter may be coupled between any two voltage domains and may receive two or more inputs upon which a logical operation and level shift is performed by the level shifter.

In some embodiments, the level shifter may be configured to receive an enable along with the input signal. For example, the gclk and En signals input to the NAND gate 30 could be incorporated into the level shifter. In such an embodiment, additional transistors may be included. Such transistors may have the enable signal or its complement coupled to the gate terminal and may be placed in parallel or series with the transistors having gate terminals coupled to the input signal or its inversion. Parallel or series coupling is selected to prevent the effect of the input signal if the enable signal is not asserted to indicate enabled.

As noted above, an additional NMOS transistor may be coupled in series with the transistor T₉ of FIG. 3 (i.e. the transistor T₂₁) or the transistor T₅ of FIG. 4 (i.e. the transistor T₁₉), with the gate terminal coupled to the V_(M) supply voltage, to make the circuit symmetrical. Similarly, various other PMOS or NMOS transistors T₁₇-T₁₈, T₂₀, and T₂₂ may be provided for circuit symmetry.

Turning now to FIG. 5, a flowchart is shown illustrating one embodiment of a method for using the V_(L)OFF signal and level shifters as described above to protect receiving circuitry such as the memory circuits when source circuitry has its supply voltage powered down and/or powered up. For example, the method may be implemented by the power control circuit 16 (in conjunction with an external voltage regulator to power the supply voltage up or down, in one embodiment).

The power control circuit 16 may monitor various signals and determine if the V_(L) supply voltage is to be powered down (decision block 70). If so (decision block 70, “yes” leg), the power control circuit 16 may assert the V_(L)OFF signal (block 72). Optionally, the power control circuit 16 may delay for a period of time to allow the V_(L)OFF signal to propagate and the level shifters to establish their predetermined voltage levels. Once the delay has expired (decision block 74, “yes” leg), the power control circuit 16 may cause the V_(L) supply voltage to be powered off (block 76). For example, the power control circuit 16 may transmit a request to a voltage regulator or other power circuit that supplies the V_(L) supply voltage. The V_(L)OFF signal may remain asserted.

At some point, it may be desirable to power the V_(L) supply voltage back up (or restore the voltage—decision block 78). Similar operation may occur at initial power up of the V_(L) supply voltage (e.g. when a device containing the integrated circuit 10 is powered on). When the V_(L) supply voltage is to be restored (decision block 78, “yes” leg), the power control circuit 16 may cause the V_(L) supply voltage to be powered up (block 80). For example, the power control circuit 16 may transmit a request to the voltage regulator or other power circuit that supplies the V_(L) supply voltage, as mentioned above. Optionally, the power control circuit 16 may delay for a time period to allow the V_(L) supply voltage to power up and stabilize, and for the circuitry powered by the V_(L) supply voltage to initialize. When the delay expires (decision block 82, “yes” leg), the power control circuit may deassert the V_(L)OFF signal (block 84), and the level shifters may begin regular operation.

In one embodiment, the input signal (In) to the level shifter 20 may be required to be stable at a value at the time that V_(L)OFF is asserted until the V_(L) supply voltage is powered down, and from the power up of the V_(L) supply voltage to the deassertion of V_(L)OFF. The power control circuit 16 may ensure the stable state of the input signal before asserting V_(L)OFF. For example, in the embodiment of FIG. 2, the gclk may be stopped at a desired level and/or the enable may be deasserted to fix the desired signal level on the input signal (In) to the level shifter 20.

Turning now to FIG. 6, a method is shown illustrating one embodiment of modifying a level shifter circuit having embedded logic such as the level shifter 20 to provide better symmetry and to support a low V_(L) supply voltage magnitude. As mentioned previously, dummy transistors may be inserted into the circuit to provide symmetry (block 90). Each dummy transistor may be sized to match a corresponding transistor in the other leg of the level shifter circuit, so that level shifter legs may end up with the same number of transistors. The gate terminal of the dummy transistor may be tied to a static voltage that ensures that the transistor is on or off, as desired.

In one embodiment, the level shifter transistors may also be employed with non-minimum channel lengths (block 92). Using non-minimum sized devices may improve the preservation of the symmetry over various manufacturing variations that may occur in various instances of the integrated circuit 10. In other embodiments, minimum channel-length devices may be used.

In one embodiment, the transistors in the level shifter 20 may be designed to senseamp design rules (reference numeral 94). The senseamp design rules 94 may include using a step layout of the two legs of the level shifter (as opposed to mirroring) (block 96). Stepping the layout, so that the corresponding transistors are oriented in the same way, may minimize the effect of manufacturing variations (e.g., if an implant is arriving at an angle, it will affect both legs in approximately the same fashion). For similar reasons, fingered transistors may employ equal length fingers (block 98). For critical level shifters (e.g. level shifters that impact the timing of the integrated circuit 10), at least two fingers may be used for each transistor (block 100). Additionally, the gain of the level shifter (e.g. the ratio of the size of the transistors in the output stage to the size of the transistors in the shift stage) may be limited to no more than 3:1 (block 102).

In some embodiments, if the logic circuitry that generates the input signal (In) may ensure that the input signal is in a particular state when the V_(L)OFF signal is asserted and that the input signal does not change state with the V_(L)OFF signal is asserted, the design of the level shifters 20 a and 20 b may be modified to remove certain transistors and occupy a smaller area, in some embodiments. For example, FIG. 7 is a circuit diagram of a third embodiment of a level shifter 20 c that is similar to the level shifter 20 b, except that the transistors T₁₅ and T₁₉ have been eliminated. Accordingly, the sources of the transistors T₅ and T₉ are coupled to ground. The transistors T₃-T₅, T₇-T₉, T₁₁, T₁₃, T₁₄, and T₁₆-T₁₈ may operate in the fashion described above for the level shifter 20 b.

Additionally, the transistor T₂₃ is provided in the level shifter 20 c, with its source coupled to ground, its drain coupled to the gates of the transistors T₈ and T₉, and its gate coupled to receive the V_(L)OFF signal. For the embodiment of FIG. 7, the input signal (In) may be held steady at a logical one when the V_(L)OFF signal is asserted. The output of the inverter 56 (while V_(L) is still powered on) is thus a logical zero, which supports the operation of the transistor T₂₃ when the V_(L)OFF signal is asserted. Once the V_(L) supply voltage is powered down, the transistor T₂₃ may retain the desired state on the node N1 (through the transistor T₈).

Turning now to FIG. 8, a fourth embodiment of a level shifter 20 d is shown that is similar to the level shifter 20 a, except that the transistor T₂₁ has been eliminated and the source of the transistor T₉ is connected to the drain of the transistor T₆. This embodiment may still retain symmetry since the nodes N1 and N2 are affected similarly by the transistor T₆. Viewed in another way, the drain of the transistor T₆ may be a “virtual ground” for both the transistors T₅ and T₉ when the V_(L)OFF signal is deasserted. FIG. 9 is a fifth embodiment of a level shifter 20 e that is similar to the level shifter 20 b, except that the transistor T₁₉ has been eliminated and the source of the transistor T₅ is connected to the drain of the transistor T₁₅. As with the embodiment of FIG. 8, this embodiment may still retain symmetry since the nodes N1 and N2 are affected similarly by the transistor T₁₅. Viewed in another way, the drain of the transistor T₁₅ may be a “virtual ground” for both the transistors T₅ and T₉ when the V_(L)OFF signal is deasserted.

FIG. 10 is a sixth embodiment of a level shifter 20 f that is similar to the level shifter 20 d shown in FIG. 8. Another embodiment may be similar to the level shifter 20 a (i.e. with the transistor T₂₁ included) as well as the additional transistors shown in FIG. 10. The level shifter 20 f includes an OutLo_H input that may be used to indicate whether the output (Out) is to be held in a low state or a high state. Thus, the level shifter 20 f may be flexible and may be used for either holding the output high or low as desired. In one embodiment, of the OutLo_H signal is asserted to a binary one, the output node Out will be high when V_(L)OFF is asserted. If the OutLo_H signal is deasserted to a binary zero, the output node Out will be low when V_(L)OFF is asserted. Other embodiments may use the opposite sense for the asserted and deasserted states.

In the illustrated embodiment, the gate of the transistor T₂₂ is coupled to the output of the inverter 50 instead of being tied high as shown in the embodiments of FIG. 3 and FIG. 8. The source of the transistor T₂₂ is coupled to the drain of a transistor T₃₀, which has its source coupled to receive the V_(M) supply voltage. The gate of the transistor T₃₀ is coupled to receive the OutLo_H signal. The source of the transistor T₂ is coupled to the drain of a transistor T₃₁, which has its source coupled to ground and its gate coupled to receive the OutLo_H signal. Accordingly, if V_(L)OFF is deasserted, both transistors T₂₂ and T₂ are off and the level shifter 20F functions to level shift the input signal. If V_(L)OFF is asserted, both transistor T₂₂ and T₂ are on. One of transistors T₃₀ or T₃₁ is on, depending on the state of the OutLo_H signal, and the node N1 is either pulled high or low.

FIG. 11 is a seventh embodiment of a level shifter 20 g. The embodiment of FIG. 11 may be similar to the level shifter 20 d shown in FIG. 8, except that the transistors T₂₀ and T₂ and the inverter 52 are eliminated. Another embodiment may be similar to the level shifter 20 a, with the transistor T₂₁ included. In the embodiment of FIG. 11, the gate of the transistor T₂₂ is coupled to receive the V_(L)OFF signal. Accordingly, if the V_(L)OFF signal is deasserted, the transistors T₁ and T₂₂ are off and the transistor T₆ is on. If the V_(L)OFF signal is asserted, the transistors T₁ and T₂₂ are on and the transistor T₆ is off. Both nodes N1 and N2 are pulled high, and the output signal Out is low. The circuit is symmetrical, and thus no dummy transistors are used in this embodiment.

Turning next to FIG. 12, a block diagram of one embodiment of a system 110 is shown. In the illustrated embodiment, the system 110 includes at least one instance of the integrated circuit 10 coupled to one or more peripherals 114 and an external memory 112. A power supply 116 is also provided which supplies the V_(L) and V_(M) supply voltages to the integrated circuit 10 as well as one or more supply voltages to the memory 112 and/or the peripherals 114. In some embodiments, more than one instance of the integrated circuit 10 may be included.

The external memory 112 may be any desired memory. For example, the memory may comprise dynamic random access memory (DRAM), static RAM (SRAM), flash memory, or combinations thereof. The DRAM may comprise synchronous DRAM (SDRAM), double data rate (DDR) SDRAM, DDR2 SDRAM, DDR3 SDRAM, etc.

The peripherals 114 may include any desired circuitry, depending on the type of system 110. For example, in one embodiment, the system 110 may be a mobile device and the peripherals 114 may include devices for various types of wireless communication, such as wifi, Bluetooth, cellular, global position system, etc. The peripherals 114 may also include additional storage, including RAM storage, solid state storage, or disk storage. The peripherals 114 may include user interface devices such as a display screen, including touch display screens or multitouch display screens, keyboard or other keys, microphones, speakers, etc.

Numerous variations and modifications will become apparent to those skilled in the art once the above disclosure is fully appreciated. It is intended that the following claims be interpreted to embrace all such variations and modifications. 

1. An apparatus comprising: a circuit supplied by a first supply voltage during use, the circuit having at least a first input signal; and a level shifter supplied by the first supply voltage during use and coupled to provide the first input signal to the circuit, wherein the level shifter is coupled to receive a plurality of second input signals, at least one of which is sourced from circuitry supplied by a second supply voltage during use, and wherein the level shifter comprises a plurality of transistors configured to generate the first input signal by logically combining the plurality of second input signals and level shifting to the first supply voltage, and wherein the level shifter comprises at least one additional transistor that is not required for functionality in the level shifter but rather is included to ensure symmetry in the level shifter by matching at least one of the plurality of transistors that is required for functionality, and wherein the additional transistor has a gate terminal tied to a static voltage during use.
 2. The apparatus as recited in claim 1 wherein the static voltage ensures that the additional transistor is off during use.
 3. The apparatus as recited in claim 1 wherein the static voltage ensures that the additional transistor is on during use.
 4. An apparatus comprising: a circuit supplied by a first supply voltage during use, the circuit having at least a first input signal; and a level shifter supplied by the first supply voltage during use and coupled to provide the first input signal to the circuit, wherein the level shifter is coupled to receive a second input signal sourced from circuitry supplied by a second supply voltage during use, and wherein the level shifter comprises a plurality of transistors configured to generate the first input signal by level shifting the second input signal; wherein the level shifter is further coupled to receive a power control signal indicating, when asserted, that the second supply voltage is to be powered down, and wherein the level shifter is configured to assert a predetermined level on the first input signal independent of the second input signal and responsive to an assertion of the power control signal, and wherein the level shifter further comprises at least one additional transistor that is not required for functionality in the level shifter but is included to provide symmetry for at least one transistor of the plurality of transistors, wherein the at least one transistor is controlled by the power control signal, and wherein the at least one additional transistor is not controlled by the power control signal but matches the at least one transistor to provide the symmetry, and wherein the at least one additional transistor has a gate terminal tied to a static voltage during use.
 5. The apparatus as recited in claim 4 wherein the level shifter comprises a first n-type metal-oxide-semiconductor (NMOS) transistor having a gate terminal coupled to receive a signal corresponding to the power control signal, and wherein the first NMOS transistor is coupled to a first leg of a shift stage of the level shifter, and wherein the at least one additional transistor comprises a second NMOS transistor coupled to a second leg of the shift stage.
 6. The apparatus as recited in claim 5 wherein the second NMOS transistor has a gate terminal coupled to receive the first supply voltage as the static voltage.
 7. The apparatus as recited in claim 5 wherein the signal on the gate terminal of the first NMOS transistor is an inverse of the power control signal.
 8. The apparatus as recited in claim 4 wherein the level shifter further comprises a first transistor coupled to a gate terminal of one or more transistors that are controlled by the second input signal, the first transistor controlled by the power control signal and configured to drive the gate terminal during a time that the second supply voltage is powered down to a state that corresponds to a state of the second input signal prior to the second supply voltage being powered down.
 9. The apparatus as recited in claim 4 wherein the level shifter comprises a first n-type metal-oxide-semiconductor (NMOS) transistor having a gate terminal coupled to receive a signal corresponding to the power control signal, and wherein the first NMOS transistor is coupled to a first leg of a shift stage of the level shifter and the second leg of the level shifter.
 10. The apparatus as recited in claim 4 wherein the level shifter is further coupled to receive a second control signal, wherein the predetermined level is responsive to the second control signal.
 11. A level shifter comprising: a first node on which an output signal of the level shifter is provided by the level shifter during use, wherein the output signal is received by circuitry supplied with a first supply voltage during use; a first input on which a first input signal is received by the level shifter during use, wherein the first input signal is generated by circuitry supplied by a second supply voltage during use; a second input on which a second input signal is received by the level shifter during use; and shifting circuitry coupled between the first input, the second input, and the first node and supplied by the first supply voltage during use, wherein the shifting circuitry is configured to level shift the input signal to generate the signal on the first node, wherein the shifting circuitry comprises one or more transistors that perform a logical operation on the first input and the second input to generate the output, and wherein the shifting circuitry further comprises one or more dummy transistors to provide symmetry in the shifting circuitry by matching the one or more transistors that provide the logic function, wherein the one or more dummy transistors do not perform the logic function on the first input and the second input, and wherein each of the one or more dummy transistors have a gate terminal tied to a static voltage during use.
 12. The level shifter as recited in claim 11 wherein the dummy transistors each correspond to a transistor in the shifting circuitry that provides the logical operation, and wherein the dummy transistors are each an opposite type of transistor to the corresponding transistor.
 13. The level shifter as recited in claim 11 wherein the shifting circuitry comprises two symmetrical series connections of transistors comprising the transistors that perform the logical operation and the dummy transistors. 